1. Field of the Invention
The present invention relates to a method and apparatus for converting the format of a digital video signal; and more particularly, to a method and apparatus for down-converting a digital video signal.
2. Description of the Related Art
Digital video signal processing is an area of science and engineering that has developed rapidly over the past decade. The maturity of the Moving Picture Expert Group (MPEG) video coding standard represents a very important achievement for the video industry and provides strong support for digital transmission of video signals. With advancements in digital compression and other techniques such as digital modulation and packetization, as well as VLSI technology, the fundamentals of television have been reinvented for the digital age.
The first U.S. digital television transmission standard developed for broadcast of high and low definition television by a Grand Alliance of companies has been proposed to the Federal Communications Commission (FCC). High definition digital television broadcasts are typically referred to as HDTV, while low definition digital television broadcasts are generally referred to as SDTV. These terms will be used throughout this application, but are not tied to a particular format or standard. Instead, these terms are used to cover the high and low definition digital television of any coding standard (e.g., such as for VTRs and television).
In 1994 SDTV broadcasts became a reality when the first digital television services, broadcasted via satellite, went on the air. The Digital Satellite Service (DSS) units developed by Thomson Consumer Electronics, etc. have been distributed to more than 1 million homes. The highly sophisticated methods of transmitting and receiving digital television not only produce higher-quality television broadcasts, but also create new services, such as movies on demand, interactive programming, multimedia applications as well as telephone and computer services through the television.
Soon, HDTV will become a reality and join SDTV. Accordingly, in the near future, expect advanced television (ATV) broadcasts which include co-existent broadcasts of HDTV and SDTV. The problem, however, arises in that the HDTV signal can not be decoded by current SDTV decoders or NTSC decoders. (NTSC is the current analog broadcast standard in the U.S.) Several efforts have been reported to address this problem. U.S. Pat. No. 5,262,854 to S. Ng proposes a hierarchical decoder structure to provide a hierarchical representation of a decoded video sequence such that multiple video resolution can be reconstructed from the compressed MPEG bit stream. A modified version of this structure has been proposed by H. Sun, an inventor of the subject application, “Hierarchical Decoder for MPEG Compressed Video Data,” IEEE Trans. on Consumer Electronics, August, 1993, Vol. 39, Number 3, pp. 559-562. Furthermore, a detailed investigation of an all-format ATV decoder has been reported by Jill Boyce, John Henderson and Larry Pearlstein, “An SDTV Decoder with HDTV Capability: An All-format ATV Decoder,” SMPTE Fall Conference, New Orleans, 1995.
Conventionally, the spatial domain down-sizing in converting from HDTV to SDTV was achieved by cutting DCT domain coefficients and performing the inverse DCT with the remaining coefficients. First, the image is filtered by an anti-aliasing low pass filter. The filtered image is then down-sampled by a desired factor in each dimension. For the DCT based coded image (such as MPEG video coding) the conventional method requires converting the compressed image to the spatial domain by inverse DCT and then filtering and down-sampling or down-converting the inverse DCT. Alternatively, both the filtering and down-sampling operations are combined in the DCT domain by cutting frequencies and taking the inverse DCT with a lesser number of DCT coefficients.
When performing, for example, MPEG video encoding of HDTV, image blocks of 8×8 pixels in the spatial domain are converted into 8×8 DCT (discrete cosine transform) blocks of DCT coefficients in the DCT or frequency domain. Specifically, in most coding formats such as MPEG, the HDTV signal is divided into a luminance component (Y) and two chroma components (U) and (V). Furthermore, instead of U and V chroma blocks, some standards use color difference signal chroma blocks. For the purposes of discussion only, U and V chroma blocks will be used. Most formats such as MPEG specify different encoding sequences. In each encoding sequence a sequence header identifies the encoding sequence. Furthermore, in each encoding sequence, macro blocks of 8×8 DCT blocks of DCT coefficients are formed.
Encoding sequences for HDTV typically include the 4:2:0 encoding sequence, the 4:2:2 encoding sequence, and the 4:4:4 encoding sequence. In the 4:2:0 encoding sequence a macro block consists of four 8×8 luminance DCT blocks, one 8×8 U chroma DCT block, and one 8×8 V chroma DCT block. In the 4:2:2 encoding sequence a macro block consists of four 8×8 luminance DCT blocks, two 8×8 U chroma DCT blocks, and two 8×8 V chroma DCT blocks. Finally, in the 4:4:4 encoding sequence a macro block consists of four 8×8 luminance DCT blocks, four 8×8 U chroma DCT blocks, and four 8×8 V chroma DCT blocks. SDTV includes similar coding sequences, but the DCT blocks are 4×4 DCT blocks.
Unfortunately, the conventional conversion techniques produce SDTV signals having a significant amount of block edge effect and other distortions. Additionally, these conventional techniques do not offer high processing speeds, and fail to produce an SDTV signal acceptable to conventional SDTV digital video decoders. Furthermore, when motion compensation must be performed, the conventional down converters require storing two complete anchor pictures in the spatial domain. Consequently, the memory requirements for conventional down converters are quite large. Because memory requirements of down converters account for a large part of the overall device cost, the large memory requirements of conventional down converters adversely impacts the cost of such devices.